Compositions that are radiation-curable have many uses. For example, it is known, as illustrated in U.S. Pat. No. 4,575,330 to C. W. Hull, to form three-dimensional objects of complex shape using ultraviolet light, such as that from a laser beam, to solidify superposed layers of a liquid ultraviolet-curable ethylenically unsaturated acrylate or methacrylate based material at the surface of a liquid reservoir of such material. Thin-walled objects, usually of honeycomb character, are formed in this manner. This method of producing objects is known as "stereolithography".
The ultraviolet dosage is limited to speed the process and limit the depth of polymerization to the layer being solidified. Even when the thin-walled object is incompletely cured and has inadequate strength and durability, the process is relatively slow because acrylate-functional materials do not respond very rapidly and the available ultraviolet lasers are weak and lose power rapidly.
More particularly, the solid objects produced by known stereolithographic processes are formed by the ultraviolet polymerization of a liquid ultraviolet-curable ethylenically unsaturated material which is acrylate or methacrylate based. Such materials have many disadvantages other than their relatively slow cure, and among these are dimensional instability on partial cure, shrinkage on cure and also their irritating effects on humans.
The ultraviolet dosage required for partial cure of the layers typically used in the Hull process, namely: those having a thickness, i.e. depth of cure, of from 1 to 10 mils, is in the range of 1 to 15 Joules per square centimeter. This is a significant amount of energy when one considers that the material has only been transformed from a flowable liquid into a lightly cross-linked solvent-swellable three-dimensional polymeric thin-walled element.
These photoformed objects are somewhat gelatinous and mechanically weak due to the low degree of cross-linking and the presence of unconverted monomers and oligomers (which are still unsaturated) within the partially polymerized polymeric structure of the object. As a result of the incomplete consumption of the ethylenic unsaturation, problems arise as how to more completely cure (thermoset) these objects after they have been removed from the bath of liquid ultraviolet-curable ethylenically unsaturated material in which they were formed.
Moreover, in removing the photoformed objects from the liquid (meth)acrylates from which they are formed, it is hard to avoid human contact with these liquids or their vapors which are toxic, odorous and irritating.
It is important to note that polymerization does not continue after the ultraviolet exposure ceases. Additionally, a more extensive ultraviolet-cure of the initially formed polymeric object is not entirely satisfactory because the ultraviolet light has difficulty penetrating into the interior of the solid objects under consideration.
A radiation-curable liquid composition which is not (meth)acrylate based and which will fully cure where polymerization is initiated without requiring an additional curing step or mechanism is highly desirable. The present invention provides such a composition, and it does so in a particularly convenient fashion using vinyl ether polyurethane compositions.
Vinyl ether-terminated polyurethanes are described in Bishop, Pasternack and Cutler U.S. Pat. No. 4,472,019 and also in Lapin and House U.S. Pat. No. 4,751,273 which issued on June 14, 1988. In each of these patents the vinyl ether-terminated polyurethane is formed by the reaction of an aliphatic monohydric vinyl ether with a diisocyanate.
In the Lapin and House disclosure, the monohydric vinyl ether is formed by a reaction of a polyol with acetylene in the presence of potassium hydroxide at elevated temperature and pressure. That reaction method significantly limits the vinyl ethers which can be produced because potassium hydroxide degrades some materials, such as ester groups, and interferes with the subsequent reaction with isocyanate. Also, and while it is broadly disclosed that one may react the diisocyanate with the monohydric vinyl ether in the presence of the other components of the reaction mixture, these other components are normally separated from the desired monohydric vinyl ether by distillation. Indeed, at least some of these other components of the reaction mixture are necessarily separated from the monohydric vinyl ether product by distillation when the desired products of reaction are separated from the potassium hydroxide catalyst by distillation.
The Lapin and House patent also employs a 1:1 stoichiometry of hydroxy and isocyanate groups, albeit in some instances it is indicated to be desirable to use a slight excess of hydroxy vinyl ether to ensure the complete reaction of the isocyanate functionality. Accordingly, if the reaction is not pushed to cause it to be complete, or if excess hydroxy vinyl ether is employed, the polyurethane product will include small amounts of unreacted hydroxy functionality. This unreacted hydroxy functionality disturbs the cationic cure in the present invention when the cationic photoinitiator concentration is very low, as is important to the formation of the relatively thick layers, i.e., layers having a depth of cure greater than about 1 mil, which are contemplated herein for uses such as stereolithography.
Lapin and House disclose the production of only one hydroxy vinyl ether, namely: triethylene glycol monovinyl ether. This distilled product is stated to have been obtained with a purity of 95 percent, so polyhydric materials have substantially been eliminated. It is this relatively pure mixture which is reacted at room temperature with a stoichiometric proportion of either 2,4-toluene diisocyanate or diphenylmethane diisocyanate, the reaction being carried out by simply stirring the mixture and the diisocyanate in the presence of dibutyl tin dilaurate catalyst for up to 5 hours without extraneous heat. As a result, the substantial elimination of hydroxy functionality is not assured. Cure is disclosed using various radiation sources. When ultraviolet light is used for cure, the coating is merely indicated to be "thin" and to have cured to produce "a tack-free glossy coating" when 4 percent of a triaryl sulfonium salt is used as the catalyst. The present invention intends to achieve far more than merely obtaining a tack-free glossy surface on a very thin film, i.e., a film having a depth of cure equal to or less than about 1 mil, as will be apparent from the discussion which follows.
Lapin and House also emphasize the production of low molecular weight divinyl ether-terminated polyurethanes. Thus, while polyols containing more than two hydroxy groups per molecule are mentioned, diols are stressed and are the only materials used in the examples. In Lapin and House, there is no contemplation of vinyl ethers in which more than two vinyl ether groups are present on the polymer which is formed.
In the present invention it is desired to obtain tough films by using higher molecular weight compositions. Higher molecular weight is preferably obtained in the present invention by having the polyurethane oligomer contain a plurality of internal urethane groups. The toughness of the cured films produced from the present compositions is also enhanced by providing vinyl ether polyurethane compositions having an average vinyl ether functionality in excess of 2.0.
Thus, and for many reasons, Lapin and House does not provide ultraviolet-curable compositions of interest to the production of the elastomeric coatings or layers which are contemplated herein. There are also many other limitations in the Lapin and House disclosure which are overcome by this invention.
Conventional compositions that utilize hydroxy vinyl ethers utilize hydroxy vinyl ethers that are highly purified, i.e., have a very high concentration of the particular hydroxy vinyl ether. These highly purified hydroxy vinyl ethers are costly to produce. Conventional methods of producing these hydroxy vinyl ethers require elevated temperatures and pressures and/or require a long reaction period.